Memory Technology: Putting the
“nano” in your iPod
Eric Pop
Dept. of Electrical & Computer Engineering
http://poplab.ece.uiuc.edu
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Applications of Memory
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1 Gigabyte = 1 GB = 1 billion bytes ≈ 10243 bytes (?!)
1 byte = 8 bits (1/0 state)
1 Human genome ≈ 0.8 GB
1 DVD ≈ 4.7 GB
1 iPod nano = 4 or 8 GB of Flash memory
1 laptop ≈ 2 GB DRAM memory
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This Is Science, Technology, and Business
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Giga-Bytes and Giga-Dollars
Over the same period, memory market accounted for 2535% of total semiconductor market. PS: cost matters!!!
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How do you cram this much
information into your pocket?!
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The Scale of Things
Source: doe.gov
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The Size of a Nanowire
nanowire
diameter ~ 50 nm
hair diameter
~ 50 µm
Source: L. Tong, http://www.nsf.gov/od/lpa/news/03/pr03147.htm
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The Abacus, Ancient Digital Memory
Roman Abacus (ca. 200BC)
Chinese Abacus (ca. 190AD)
• Information represented in digital form
• Each rod is a decimal digit (units, tens, etc.)
• Finite number of states for each bead
• A bead in the abacus is a memory device, not a logic gate
Sources: R. Cavin (SRC), Wikipedia
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More on Ancient History
Source: C. Webb, IEDM 2001
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Memory in a Modern Computer
$$$$
$$$
$
¢
Cost
Capacity &
Latency
Source: Brewer, IEEE Circuits and
Devices Mag., Apr. 2005
Modern CPU:
• Operate at GHz (nanosecond access times)
• Modern memory vary from ~1 ns access
times (SRAM) to 1 ms access times (hard
disk)
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Another Look at the Memory Hierarchy
Source: R. Cavin (SRC)
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From Silicon to Bits
Vertical
Cross-section
Packaged die
8-12” silicon wafer
(100s of dies)
Single transistor
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On-Chip SRAM Cache Memory
Lots of SRAM cache
(static random-access memory)
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Managing Billions of Bits
Memory:
– Volatile vs. Non-volatile
(charge stored when power
is off)
– Read-only vs. Read-write
– Charge based vs. ???
– Fast vs. Slow
– Large bit vs. Small bit area
– Cheap vs. Expensive
Source: Stanford EE313 Notes
http://www.stanford.edu/class/ee313/
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SRAM = Fastest, Largest, Volatile (6T)
• Two inverters in series
(“stuck” in either 1 or 0
stable states) + two access
transistors
• Six transistors total (6T)
• Volatile! Bit state (0/1)
maintained only while
power is ON
• Constant leakage current
must be replenished 
power dissipation (~ 10 mA
standby for 1 Mb cell array)
• 6T SRAM is BIG: ~100 F2
• Fast: < 1 ns access time!
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SRAM = Fastest, Largest, Volatile (6T)
• Two inverters in series
(“stuck” in either 1 or 0
stable states) + two access
transistors
all total
SRAM
• SixAlmost
transistors
(6T)is
• embedded
Volatile! Bit state
(0/1)
within
micro-
maintained only while
processors
power
is ON these days
(why?)current
• Constant leakage
must be replenished 
power dissipation (~ 10 mA
standby for 1 Mb cell array)
• 6T SRAM is BIG: ~100 F2
• Fast: < 1 ns access time!
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DRAM = Fast, Small, Volatile (1T-1C)
64 Mbit DRAM from 1996
http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_5/illustr/i5_1_1.html
Storage
capacitor
Word line
Access transistor
C
Access FET
Bit line
• Bit 1/0: Charge stored on capacitor
• Access (~10 ns) with one transistor
• “1T-1C” configuration
• Read is destructive: charge spills out of
Trench
Capacitors
C ≈ 25 fF
capacitor, must be recharged
Area: 8-10 F2
• Refresh charge every 64 msec
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DRAM = Fast, Small, Volatile (1T-1C)
64 Mbit DRAM from 1996
http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_5/illustr/i5_1_1.html
Storage
capacitor
Word line
Access transistor
C
Access FET
Bit line
Trivia: DRAM capacitance
cannot be miniaturized further
due to cosmic ray damage!
Trench
Capacitors
C ≈ 25 fF
Area: 8-10 F2
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FLASH = Slow, Small, Non-Volatile (1T)
NOR: “Lucky” electron programming
Drain current ~ 50 mA
• Charge stored by “tunneling” within single transistor (1T)
(yes, this is quantum mechanics storing the Maroon 5 song
bits on your iPod nano)
• Non-volatile, but relatively slow (~100 ns – 1 µs)
• A single bit ~ 100 stored electrons today (2008)
• Very, very hard to make this smaller, or charge will leak out!
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Flash Layout: NOR vs. NAND
• How do you get more speed out
of Flash?
• “NAND” layout is slower but
denser  used for data storage
(iPod nano, USB drives, etc.)
NOR: 16 Cells
• “NOR” layout is faster but 2x
larger (less dense)  used in cell
phones, code storage
NAND: 16 Cells
• Data access speed also depends
on the bit (cell) layout
• NOR read ~10s of ns
• NAND read ~1 µs
Source: Al Fazio, Intel
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Phase-Change = Fast, Small, Non-Volatile
• Can we store memory without storing electron charge?
• Think about CD/DVDs, magnetic hard drives, even vinyl records
• DVDs: “phase-change
material” layer changes from
crystalline to amorphous
when heated by laser beam
• 10 ns access time, very
small footprint
0.3
Current (A)
• Same material (GeSbTe)
may be used for very small
and fast electrical memory
(b)
0.2
Data "0"
low-R (fcc)
0.1
Data "1"
high-R (a)
VTH
0
0
1
2
Voltage (V)
3
4
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Intel, Samsung, UIUC Hard at Work…
Put in perspective:
NAND Flash chips of
8+ Gb in production
“Samsung completed the first working prototype of what
is expected to be the main memory device to replace
high density Flash in the next decade – a Phase-change
Random Access Memory (PRAM). The company unveiled
the 512 Mb device at its sixth annual press conference
in Seoul today.” (September 2006)
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Questions?
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